Characterization of Rice Husk and the Crystallization ... · PDF filecombustion, a rice husk ash rich in silica is obtained, with some ... LACCEI International Multi-Conference for

14th

LACCEI International Multi-Conference for Engineering, Education, and Technology: “Engineering Innovations for

Global Sustainability”, 20-22 July 2016, San José, Costa Rica.

Characterization of Rice Husk and the Crystallization

Process of Amorphous Silica from Rice Husk Ash Ana Rivas, PhD1, Grace Vera, Eng.2, Julio Caceres, Eng.3,Víctor Palacios MSc.4, Andrés Rigail PhD 5 Mauricio Cornejo PhD6

1 Escuela Superior Politécnica del Litoral, ESPOL, FIMCP, Campus Gustavo Galindo Km 30.5 Vía Perimetral, P.O. Box 09-01-

5863, Guayaquil, Ecuador. Universidad Simón Bolívar-Venezuela. [email protected] or [email protected] 6ESPOL-Ecuador. [email protected]

2 ESPOL. [email protected]. [email protected], Espol. [email protected]. ESPOL. [email protected].

Abstract– Rice husk (RH) is a rich silica by-product of rice

production with great potential for industrial applications. After

combustion, a rice husk ash rich in silica is obtained, with some

amount of other inorganic components constituted mainly of

alkaline elements. The study was conducted to characterize both

the rice husk and rice husk ash (RHA) in order to evaluate their

constitution and the form in which mentioned impurities are

present. The amorphous silica crystallized when heated under

certain conditions. In this way, the examination of the

crystallization process of rice husk ash with different impurity

levels was also carried out by varying the temperature, time and

heating conditions. The study reveals a preferential distribution of

silica at the cell tip of the outer RH surface cells. The impurities

form compounds with combinations of different elements such as

Ca and Mg, P and K, and are mainly localized below the inner

surface in the form of spherical and cuboidal shapes, as well as

irregular morphologies.

Digital Object Identifier (DOI):

http://dx.doi.org/10.18687/LACCEI2016.1.1.093

ISBN: 978-0-9822896-9-3

ISSN: 2414-6390

http://dx.doi.org/10.18687/LACCEI2016.1.1.093

14th LACCEI International Multi-Conference for Engineering, Education, and Technology: “Engineering Innovations for

Global Sustainability”, 20-22 July 2016, San José, Costa Rica. 1

Characterization of Rice Husk and the Crystallization

Process of Amorphous Silica from Rice Husk Ash

Ana Rivas, PhD1, Grace Vera, Eng.2, Julio Caceres, Eng.3,Víctor Palacios MSc.4, Andrés Rigail PhD 5 Mauricio Cornejo PhD6 1 Escuela Superior Politécnica del Litoral, ESPOL, FIMCP, Campus Gustavo Galindo Km 30.5 Vía Perimetral, P.O. Box 09-01-

5863, Guayaquil, Ecuador. Universidad Simón Bolívar-Venezuela. [email protected] or [email protected] 6ESPOL-Ecuador. [email protected]

2 ESPOL. [email protected]. [email protected], Espol. [email protected]. ESPOL. [email protected].

Abstract– Rice husk (RH) is a rich silica by-product of rice

production with great potential for industrial applications. After

combustion, a rice husk ash rich in silica is obtained, with some

amount of other inorganic components constituted mainly of

alkaline elements. The study was conducted to characterize both

the rice husk and rice husk ash (RHA) in order to evaluate their

constitution and the form in which mentioned impurities are

present. The amorphous silica crystallized when heated under

certain conditions. In this way, the examination of the

crystallization process of rice husk ash with different impurity

levels was also carried out by varying the temperature, time and

heating conditions. The study reveals a preferential distribution of

silica at the cell tip of the outer RH surface cells. The impurities

form compounds with combinations of different elements such as

Ca and Mg, P and K, and are mainly localized below the inner

surface in the form of spherical and cuboidal shapes, as well as

irregular morphologies.

. I. INTRODUCTION

Rice is harvested in large amounts worldwide, being the

world paddy production in 2015 around 740 million tonnes.

Rice hush (RH) represents about 20% of the rice grains, from

which near 30 million tonnes of ash is feasible to obtain (20%

of rice husk). Rice husk, besides organic components

constituted by cellulose, lignin and hemicellulose, also

contains around 20% of silica (SiO2). The combustion of

organic components at temperatures between 550 and 700°C

leads to rich silica ash (SiO2) in an amorphous state. In this

regard, unique characteristics of rice husk ash (RHA), such as

high silica contents (87– 97 wt. % SiO2), high porosity, low

density, high surface area and low thermal conductivity makes

it an alternative material for a variety of industrial applications

[1]. During heating, the amorphous silica becomes crystalline, and the transition temperature for this transformation, as well

as the type of phases formed, are highly affected by the level

of impurities present in the RHA. The impurities commonly

present in RHA are K, Ca, P and Fe in oxide form. These

substances promote the formation of cristobalite and trydimite

[2, affecting the structural transformation of silica from

amorphous to crystalline state [3. Additionally, the

composition of RH is not unique, it varies depending on paddy

type and source, soil fertilizers, climate conditions and sample

preparation, among others [4.

Silica in the amorphous state is preferred for some

potential applications of RHA, such as nitrides, carbides and

other inorganic materials [5. Thus, for those applications, it is

of great interest to establish the most favorable conditions to

keep the RHA silica in the amorphous state. In recent studies

[6], the authors evaluated the evolution of silica crystallization

by continuous heating in situ from room temperature to

1450°C, highlighting the effect of impurity levels in this

process. The transition temperature could also be affected by

the heating mode. In this regard, the main purpose of this study

is to evaluate the effect of isothermal heating on the

amorphous- crystalline transformation of amorphous silica rich

RHA. Moreover, the influence of different impurity

concentration levels on amorphous- crystalline transformation

will be assessed and the results are compared with those

previously obtained by dynamic in situ heating. Furthermore,

given the importance of the initial features of the rice hush on

the amorphous to crystalline transformation, especially the

impurities present in the material, a characterization of the

samples was also performed by Fourier Transform Infrared

(FTIR) Spectroscopy, Scanning Electron Microscopy (SEM)

and X-Ray diffraction (XRD). The emphasis in the

characterization was placed on the silicon distribution and type

of impurities in the samples.

II. EXPERIMENTAL PROCEDURE

The procedure followed to prepare the samples is showed

in Figure 1. Rice husk, collected from a rice mill in the region

of Taura, Guayas-Ecuador, was washed in distillated water and

dried. A portion of this sample was then pulverized to a

granulometry lower than 40µm, from which some samples

were prepared for Fourier Transform Infrared (FTIR) analysis,

with a Thermo Scientific Nicolet iS10-FTIR Spectrometer.

Additionally, A single unit of rice hush in the as-received

condition was analyzed by scanning electron microscopy

(SEM), using a FEI- Inspect and Phenom 1255 microscopes.

A fraction of the washed and dried sample was directly

calcinated at 650°C during 3h, and a second one was purified

previous calcination, in a 1NHCl water solution during 16h.

After the calcination process, these samples, labelled Sample 1 Digital Object Identifier (DOI): http://dx.doi.org/10.18687/LACCEI2016.1.1.093ISBN: 978-0-9822896-9-3ISSN: 2414-6390

mailto:[email protected]


mailto:ESPOL.%[email protected]


mailto:Espol.%[email protected]

mailto:Espol.%[email protected]



and Sample 2, respectively, were analysed by a SEM Phenom

1255 microscope. Subsequently, both samples were subjected

to static annealing to evaluate the crystallization process of the

amorphous silica present in RHA. This heat treatment was

performed in a furnace at temperatures between 800 and

1000°C during 2 and 6h, in order to evaluate the effect of

temperature and soaking time on crystallization. After

annealing some samples were analysed by X-ray diffraction

(XRD), using a Panalytical X'Pert (40 kV, 30 mA) X-ray

diffractometer. Scans were taken with a 2θ, step size of 0,017

and a counting time per step of 0,1s using a Cu-Kα radiation

source (λ=1.542 Å). The isothermal crystallization results were

then compared with those obtained by continuous heating

carried out in situ, from room temperature up to 1450°C [6].

The constituents of the samples under study are displayed

in Table 1. Apart from silica, the main components of the

RHA are K2O, P2O5 and Ca0 in amounts of 5.42, 2.95 and

2.77, respectively. Other impurities of S, Mn, Fe, Mg, Al, Cl

and Zn are also present in minor amounts. The sum of these

impurities altogether is 1.44% for Sample 1 and 0.36% for

Sample 2.

TABLE 1. Percentage of constituents on samples under investigation

Component

Weight (%)

Wt. (%)

Sample 1 Sample 2

SiO2 87.39 96.94

K2O 5.42 0.09

P2O5 2.95 1.94

CaO 2.77 0.67

SO3 0.48 -

MnO 0.28 0.02

Fe2O3 0.24 0.14

MgO 0.22 -

Al2O3 0.09 0.20

Cl 0.10 -

ZnO 0.03 -

Figure 1. Scheme of experimental procedure followed in this study.

III. RESULTS AND DISCUSSIONS

A. Fourier Transform Infrared Spectroscopy

The FTIR patterns for samples of RH, are shown in Figure

2. These pattern display bands in the interval of wavenumbers

between 500 and 4000 cm -1, the results coincide with those

reported in the literature [7]. The band with wavenumber 3402

cm-1, located at the OH stretching zone, corresponds to the

silanol (SiOH) group. The band centered at 2916 cm-1

corresponds to the methyl group attributed to the presence of

lignin [7]. The stretching of aromatic rings are visualized by

the wavenumber 1426 cm-1, while the groups Si-O-Si can be

seen at 1100 cm-1 [8]. The bands centered at 800 and 464 cm-

1are associated to the Si-H bonds [9].

Figure 2. Curves of Fourier Transform Infrared

for rice hush in the as-received condition.

B. Scanning Electron Microscopy (SEM) of RH in the as-

received condition and RHA in the calcination condition

Figure 3 shows a SEM image of a RH sample in the as-

received condition, showing both its external (a) and internal

(b) surfaces. The outside surface, named exocarpo, is rough and

has a characteristic wavy pattern, constituted by a

symmetrically arrange of cones disposed in files along the

whole surface. Each cone represents a cell [10]. Backscattered

images (BSC) in figure 3C gives details of the RH outer zone.

This figure shows the separation zones between cone files

which are constituted mainly by organic material. Particular

features of the inner zone are displayed in figure 3D. It is

comprised by a vein-like structure, regularly spaced along the

whole area with dispersed particle in between them. Figure 4

exhibits the EDS mapping analysis of silicon superimposed to

SEM images taken at the outer (Fig. 5A) and inner (Fig. 5B)

surfaces, together with oxygen mapping, figures 5C and 5D,

respectively. These elements in the external surface are mainly

distributed on the cells tips. This is due to the fact that silicon is

not present in RH in an elementary form, but rather as SiO2. On

the other hand, at the inner surface both elements appear

disperse everywhere, but with a weak signal.

Rice Hush (RH)

Washing and dried

Purification

RHA

Cristalización SiO2

Chemical Treatment

1NHCl for 16h. XRanalysys

Calcination

(Obtaining RHA)

Static heating, varying

temperature and soaking time.

Characterization by DRX

T: 650°C; t: 3h

Analysis by SEM and XRD

Analysis by FTIR, and

SEM



Figure 3. SEM image of RH in the as-received condition. a) general view. b and c) Details of external and inner surfaces, respectively.

Figure 4. Silicon and oxygen maps on the outer and inner surfaces of RH, a) Outer surface SEM image with the mapping of Si superimposed

to it. b) Oxygen map in the same area than Si in a). c) Inner surface SEM image with the mapping of Si superimposed

to it. d) Oxygen map in the same area than Si in c)

a

b c

b

c

a

d



This suggests lower concentrations of silica in the interior of

the RH. The distribution of silica in RH results is coincident

with the reported by other researchers [11]. The silica together

with lignin, provides resistance and rigidity to RH [12, 13],

protecting the RH from the surroundings.

Figure 5 shows a series of particles located at the outside

and inside of RH. They have cuboidal, squares and spherical

shapes, and are found on the top and below the inner surface.

The particle marked with number 1 in Figure 5a, seems to be

part of the inorganic components of the RH. Its EDS analysis

displays in Figure 5d, shows a high carbon peak with no

detection of any impurity elements. Figure 5b, shows a zone

where the skin layer of the inner surface was missing. This

allows the observation of a significant number of particles

inside the cavities at the interior of RH. Some of them are seen

in detailed in Fig 5c, corresponding to the area enclosed in

circle in Figure 5b. The chemical composition of squares and

spherical particles in this Figure 5c, are showed in Figures 5e

and f, respectively. Square particle 1 of Figure 5c, is rich in

Ca, and particle 2 of spherical shape has Mg, P and K. The

size of these spherical particles, also found over the inner

surface, are lower than 3m, while the ones rich in Ca have

sizes much greater. Given the distribution and location of

impurities, the results suggest that for a more efficient

purification of RH, it is recommended to grind the RH in order

to break the inner surface layer, and expose the impurities to

the chemical solution.

v

Element

Symbol

Atomic

Concentration

Error

C 61,5 0.8

O 35,5 0.2

Si 3 0.1

Element

Symbol

Atomic

Concentration

Error

O 44.3 0.1

C 39.8 0.7

P 1.4 0.4

Mg 1.6 0.2

Si 0.6 0.4

K 0.4 2.0

N 11.9 1.9

Element

Symbol

Atomic

Concentration

Error

O 48.8 0.1

C 46.2 0.9

Ca 3.1 0.3

Si 1.8 0.2

Figure 5. SEM images showing some components of RH with their respective EDS analysis, a) Inner surface.

b) Interior of sample showing below inner surface of RH. c) Detail of zone in Figure b enclosed in a circle.

d) EDS analysis of particle 1 enclosed in a circle in image a. e) and f) EDS analysis of particle 1 and 2 in image c, respectively.

a b c

d e f

1

2 1



Figure 6 is an SEM image showing a general view of the

RHA sample, obtained by calcination of RH at 650°C for 3h.

In this figure, the outer, inner and transversal surfaces of RHA

can be recognized. The RHA conserves the RH shape which is

delineated by a rich silicon skeletal structure. The

aforementioned structure seems to remain intact after burning

the RH organic components, due to the rigidity of the material.

The densest zone of the RHA seems to be the outer part,

distinguished by a regular waved zone that corresponds to the

cells structure in the RH. This zone is detailed in Figure 6b,

with non-open porous characteristics. This result is in contrast

with the structure morphology of the depressed area that

separate the files of cells in the RH, which is highly porous in

nature after burning (Fig 6c). This part in the RH was mainly

comprised by organic material, such as was showed by Figure

4e. The inner zone of the RHA is also porous. The evidence of

these features are better observed at the cross section of the

sample, which appears in Figure 6a. Such porous structure is

accentuated by the removal of RH organic components by

burning during calcination. The inner zone of the RH is

mainly constituted by organic materials and therefore, have a

lower concentration of silica (Fig. 4b). Thus, the only parts of

the surface that seem to be dense are the cell tips, which are

zones with less organic material (See Figure 4a).

Figure 6. SEM images of Sample 1(calcinated without previous chemical treatment in 1NHCl solution).

a) General view. b) Outer surface; C) Detail of outer surface and c) Inner surface.

a

b

c

d



C. X ray diffraction of RHA in the calcination condition and

after isothermal heating

Figure 7 contains the XRD pattern of RHA of samples 1

and 2. These patterns show a XRD profile typical of

amorphous materials. Thus, at the calcination conditions

(650°C and 3h), the RHA, abundant in silica, conserves the

amorphous structure of RH, which corroborates it amorphous

nature. However, it is important to notice the difference of the

diffraction peak amplitude between these two samples and

also the small shift to the 2 position of the diffused peak. In

Sample 1, the peak measured at the half height of the peak is

narrower than in Sample 2, and the diffuse peak is shifted to

lower 2 position. In RH, the cellulose, found in RH is in

amounts around 33% [14], is the only component being

reported to have a crystalline structure as a consequence of

the interactions between Van der Waals forces between

adjacent molecules [14]. The diffraction peaks of the cellulose

are located at 2 equal to 16°, 22° y 35°, being the highest

intensity peak at 22° [69]. Those peaks overlap with the

background generated by amorphous components. In this

case, it was possible to contrast the peaks by chemical

treatment in a 4%NaOH alkaline solution [15]. However, the

cellulose decomposes in a temperature range between 250 and

350°C. Therefore, the different features in XRD peaks

cannot be attributed to the presence of cellulose. The basic

difference between Sample 1 and 2 is the chemical

composition which includes the carbon content and

impurities. Therefore, it is possible that the cause for the

difference in amplitude and 2 position of the diffuse peak

originates form differences in chemical composition in the

samples.

Figure 7 XRD patterns of Sample 1 and 2 in the calcinated condition

Figure 8 and 9 display the XRD patterns of samples

isothermally heated from 800 to 1000°C for varying times.

Sample 1 exhibits partial crystallization after exposure at

800°C for 6h and undergoes complete transformation at

900°C within 2 h. The crystallized state is composed of two

phases, cristobalite and tridimite. On the other hand, in Figure

8 it can be seen that Sample 2 remains amorphous after 6h of

heating at 1000°C. Sharp peaks around 44.7° and 65° are

displayed in this figure. They are present in both samples and

also in the original RHA (Figure 7), but the peak intensities

become more pronounced after heating. However, the phase

that leads to the appearance of these peaks decomposes with

time at higher temperatures since the XRD peaks vanished

after annealing the sample for 6h at 1000°C (Figure 9).

Although the phase that originates these peaks has not been

identified, it is probably related to a complex phosphate since

it is the major impurity in Sample 2. These findings, however,

have not been reported in the literature and invites for further

investigation.

Figure 8. XRD patterns of Sample 1 isothermal

annealing at 800°C, 900°C and 1000°C for 2 and 6h.

Figure 9. XRD pattern of Sample 2 after



Isothermal annealing at 800, 900 and 1000°C for 6h.

Table 1 shows the difference in chemical composition of

the samples under investigation. The main impurity in Sample

1 is K, found in a dioxide form; its concentration in Sample 1

is 5.42%, while in Sample 2 is less than 0. 10%. As previously

pointed out, the presence of alkaline elements promotes

crystallization of amorphous silica at lower temperatures [16].

This behavior is due to their decomposition at low

temperatures, which may produce low melting eutectic

compounds with silica, thus favoring crystallization.

Therefore, the results obtained upon isothermal heating are

consistent with those expected. Sample 1 with higher amounts

of impurities exhibited a transition from amorphous to

crystalline phase at much lower temperatures than Sample 2.

Moreover, at high temperatures the content of low melting

impurities such as those based on potassium may decrease,

which may raise the crystallization temperature.

Comparing these results of the crystallization

temperatures of samples subjected to isothermal heating with

those obtained by dynamic heating, it is found that the

crystallization of amorphous silica, beside time and

temperature, is also dependent on heating condition. It was

found in previous work [6] that in Sample 1, the first trace of

crystallization appears at 900°C and became entirely

crystalline at 1200°C, when heated continuously. On the other

hand, in isothermal annealing, it starts to transform from

amorphous to crystalline after 6h at 800°C and the

transformation is completed after 2h at 900°C. In contrast,

Sample 2, begins to crystallize at temperatures greater than

1000°C in both continuous and isothermal heating.

Therefore, in contrast with Sample 1, Sample 2 has an

isothermal starting crystallization temperature above 1000°C,

similar to dynamic heating. This results could be associated

with the increase in sample purity of Sample 2 at high

temperatures, probably due to the decomposition of some

impurity compounds present in the sample and subsequent

evaporation. Such could be the case of the phase associated to

the sharp peak located at 44.7° and 65° that disappears at

1000°C (Figure 9). It has been demonstrated that the higher

the purity level of RHA, the higher the transition temperature

from amorphous to crystalline is.

CONCLUSIONS

The silicon in RH is mainly concentrated at the tips of the

cells located at the RH outer surface, with less content of

organic material.

The impurities in the RH sample are located at the inner

surface and in the interior, they exhibit different shapes and

chemical compositions.

The crystallization process of RHA amorphous silica

depends on its chemical composition, time, temperature and

heating condition. The sample with greater levels of

impurities and static heating, crystallize at lower temperatures.

The crystallization temperature of amorphous RHA is raised

with the increase in purity levels, while heating conditions do

not show a remarkable influence.

ACKNOWLEDGMENT

The authors are thankful to the following institutions in Ecuador:

Prometeo program of SENESCYT for sponsoring the present research;

LEMAT- ESPOL for the support with the experimental work and X-Ray

Diffraction; ANDEC S.A for the X-ray Fluorescence analysis and to

Nanoinstrumentos for SEM + EDS analysis.

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Characterization of Rice Husk and the Crystallization ... · PDF filecombustion, a rice husk ash rich in silica is obtained, with some ... LACCEI International Multi-Conference for